ITIS - Intelligentes THz Interface Scanning

In this project a novel measurement method for continuous measurements of the temporal position and amplitude of a terahertz (THz) pulse was developed. The advantage of this approach is a reduced sampling time of ~20ms and an increased resolution of less than 10µm. The method is therefore very well suited for online measurements in production processes to monitor the thickness, interface position and inhomogeneities of the composition in non-conducting materials.  (FFG Project Number 838830)

Figure 1: Schematic drawing of the modified THz TD setup.
Figure 2: Fast time delay (moving coil actuator) for scanning the THz pulse.

For this project a fast oscillating mirror mounted on a moving coil actuator was integrated into a standard THz time-domain (TD) setup for transmission measurements. This additional delay line was developed in cooperation with i-RED GmbH. The data acquisition and feedback control of the slowly moving mechanical stage was built around a real-time PXIe system from National Instruments.

Figure 3: Typical time-resolved THz signal. The THz pulse occupies only a small portion of the overall measurement window.
Figure 4: Schematic of the data acquisition, feedback controller and communication between the measurement instruments.

The temporally modulated signal was analyzed with a lock-in amplifier from Anfatec which can analyze the input signal at the carrier frequency and any harmonic simultaneously. From the 1st harmonic the pulse amplitude can be determined, while the 2nd harmonic is used to monitor the position of the THz pulse.

Figure 5: Comparison of the “normal” THz signal (blue), the 1st (red) and 2nd (green) harmonic of the lock-in signal.
Figure 6: Measured thicknesses of various layers of low-density poly-ethylene foil (~ 55 µm / layer). 1, 2 or 4 layers where inserted into the THz beam path, the green horizontal line represents the reference position.

As a result the thickness of a thin polymer foil was determined with the ITIScanner. The time trace shows the continuous values of the measurements, while individual layers are added/removed from the THz beam path.